"What is" followed by plural noun

I've noticed this structure in one of the GMATPrep questions:

What is difficult to determine is the exact location of the house, the reason it was constructed, and the number of people it could accommodate.

The example is made up, but reflects the exact same kind of structure.

Since the subject is singular, the first verb obviously has to be singular. Now what is interesting is that the second verb which is singular is followed by a list of objects which is equivalent to a plural noun. Normally, a verb referring to a plural object should also be plural; this case is an exception.

Now I wonder if this exception is specific to the "What is" structure or there is a more general rule for this kind of exception.

Any thoughts are welcome.

P/S: if someone can find the GMATPrep question that uses this structure, feel free to share a link... Sorry I don't remember the other details of the question.